FIG. 1 shows an example of a tripping torque setting mechanism in a conventional overload clutch. It comprises an adjusting nut 63 which is adapted to be tightened, so as to obtain a desired value of the tripping torque, along an external thread provided on the outer surface of a cylindrical part 62 of a hub 61.
In Japanese Utility Model Application Laid-open No. 3130/1989, there is disclosed a mechanism in which setting of the tripping toque may be easily made. As illustrated in FIG. 1, the mechanism comprises a helical line 68 provided on the outer surface of the hub 61 such that the pitch of said helical line 68 coincides with that of said thread on the hub 61. The indicator 67 is provided on the nut 63. The tripping torque may be read on the basis of the rotational angle of the nut 63, namely, the positional relationship between the edge of the indicator 67 and the figures (i.e. numbers) represented along the helical line 68 so as to indicate the rotational angle.
However, as mentioned before, the pitch of the helical line 68 must be identical with that of the thread, and the figures as well as the marks for indicating the rotational angle are required to be indicated between two adjacent portions of said helical line 68. The spacing between said portions is, however, very small. Thus, in actuality, it may be difficult to determine to which portion of the two the indicator 67 corresponds.
Especially, in the event the spacing between the adjacent lines is small and the number of turns of the helical line 68 is relatively large, the corresponding relationship between the line and the figures becomes harder to determine, which may lead to an error in setting the tripping torque.
Furthermore, the indicator 67 is usually made of a bolt or a pin which is screwed or hammered into the pressure plate or the adjusting nut 63 so as to be fastened tightly thereto. Such being the fixing structure of the indicator, the indicator 67 is susceptible of loosening, which may lead to inaccurate reading of the indicator.
In addition, so as to prevent the indicator 67 from loosening, bonding by adhesives is usually employed. However, prevention of loosening by means of adhesives is not necessarily complete against vibration, etc. Needless to say, in case of breakage of such an indicator, replacement is virtually impossible.